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Anton Paar: SurPASS: Zeta Potential of contact lenses

Anton Paar: SurPASS: Zeta Potential of contact lenses

The special sample holder for soft contact lenses is designed to make sample mounting simple and reproducible on the SurPASS Electrokinetic Analyser.

Soft contact lenses have to comply with different demands regarding oxygen permeability, wettability and wearing comfort. Biocompatible materials such as phoshorylcholin or polyvinylalcohol should furthermore diminish the formation of biofilms and bacterial adhesion on the surface in order to reduce the risk of eye infections.

Zeta potential measurements provide a tool to detect the deposition of proteins or biofilm formation on the surface of contact lenses – and can be performed on the SurPASS Electrokinetic Analyser with the special holder for soft contact lenses.

New contact lenses, both for monthly and daily use, show an isoelectric point at pH 4 (IEP = pH value where the zeta potential is 0 mV), which is characteristic for an inert polymer surface. Upon wearing monthly lenses for the proposed time, the isoelectric point shifts to pH 4.7, mainly due to deposition of proteins from the tear fluid such as albumin (IEP 4.7), but also lactoferrin (IEP 8.5) or lysozyme (IEP 11.1).

The effect of wearing disposable contact lenses for daily use on the zeta potential is less pronounced. Both new and used lenses show a plateau of negative zeta potential in the basic pH range. The negative surface charge arises from the competitive adsorption of water molecules and hydroxide ions. A tear film adsorbed on the contact lens surface decreases the negative zeta potential due to the increased adsorption of water.

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